Superconductor wire is commonly formed of one or more fine filaments of superconductor composition (e.g. NbTi or Nb3 Sn) which are embedded in a copper or copper-based matrix. A typical processing sequence for producing a NbTi multifilament conductor using a hex stack, starts with a can of copper which is fabricated by back extruding a copper pipe 12 inches in diameter and 36 inches long. A cone is fixed to one end and the interior of the can is filled with hexagonal rods. The rods are a composite with a round core of NbTi and a jacket of copper. The rods are assembled into the can in a hexagonal array to completely fill the interior with a minimum void volume. The spaces between the ID of the can and the hex stack, which are too small to accommodate a full hex, are filled with partial hexes of copper cut to fit the spaces.
A lid is pressed onto the can and the can is evacuated and welded by an electron beam welder. The assembly is then extruded from the 12 inch diameter to about 3 inches. The rod thus formed is over 40 ft long and is then drawn in steps of about 20% area reduction to final size. At various places during the drawing process, the rod/wire is heat treated to cause precipitation, which increases the current density of the final wire.
The NbTi hex composite rods are themselves fabricated by extrusion. Typically, an eight-inch billet of NbTi is inserted into a copper can that is sealed as above and extruded. The resultant rod is drawn and hexed at the desired size without any intermediate heat treatment.
An example of a typical process for the manufacture of a multifilamentary Nb3Sn conductor, begins with the drilling of a plurality of holes in a Cu/Sn bronze billet for the insertion of Nb rods. This billet is then extruded to a rod, which is then drawn down to a fine wire. In some cases it is desirable that even more filaments be produced; this can be done by cutting the rod into a large number of equal lengths at some intermediate size, inserting these into an extrusion can, extruding this assembly and drawing the result to fine wire. The extrusion can in this case is either copper with a Nb or Ta barrier to prevent Sn diffusion, or bronze. The rod may be drawn through a hex-shaped die prior to cutting; if the rod is thus hexed, the lengths pack together in the extrusion can with less wasted space. In some cases it is desirable that there be provided a quantity of pure copper of good electrical conductivity. This may be done by lining a copper extrusion can with a layer of metal such as Ta or Nb which (in the case of Nb3Sn) is impermeable to tin, during high temperature heat-treatment, so that the tin does not diffuse into the copper and lower its conductivity; tantalum is the metal most commonly used. See, e.g., U.S. Pat. No. 3,996,661.
The copper to superconductor ratio of a superconductor wire (expressed as the ratio of area of copper to area of superconductor “Cu/SC” in a cross-section of the wire) is an important parameter related to stability. A quantity of a good electrical conductor in close proximity to the superconductive material is useful as an alternate current path or shunt in situations where it is likely that some fraction of the superconductive filaments will return to the normally-conducting state, which can happen, for example, in a rapidly-varying magnetic field. If in the initial phase of manufacture of the wire all of the required copper is included in the original billet, the cost of the process is very high. In addition, the processing becomes far more difficult if not impossible. To produce a superconductor with a large amount of copper is difficult because of the possibility of center burst. Center burst is the occurrence of broken filaments in the center of the composite. Center burst occurs during wire drawing if the ratio of soft matrix (copper) to hard filaments (e.g. NbTi) is too high. By maintaining this ratio low, i.e., by using a low amount of copper during the initial fabrication steps, one can avoid this problem. The alternative is to add additional copper at the final stages of fabrication. Various means have been devised to clad this additional copper in the final manufacturing step. These include:                a) Soldering the low copper to superconductor ratio wire into a copper channel. For example, a superconductor wire (core wire) with a ratio of 1:1 copper to superconductor can be converted to an 8:1 ratio by soldering the core wire into a copper channel.        b) Hot cladding on a cladding line.        c) Cladding on a tube mill.        d) Cabling copper wire around a core wire of superconductor.        
There are disadvantages to all of the above processes. Soldering precludes further processing due to the weak bonding and because of the low melting point of the solder. Further hot processing cannot be done such as FORMVAR insulating. (FORMVAR® is a registered trademark of Chisso Corporation for modified resins, used here for insulating films.) The solder would melt causing considerable problems. Hot cladding subjects the superconductor to very high temperatures in order to create a copper to copper bond. This bond is not always of the quality required and the high temperature may reduce the current density of the conductor. Cladding on a tube mill does not form a metallic bond between the copper tube clad over the superconductor core. Even with extensive redrawing, a true metallic bond is not formed and thus redrawing can subject the core to various problems such as center burst. Cabling usually requires soldering with the limitations noted above, as well as damage to the core wire if the cable is subjected to any reduction or shaping operations.